The results showed that the vaccine worked as planned. "Fewer of the people who were vaccinated got flu than the people who weren't vaccinated," said Gilbert. "We did get an indication that the vaccine was protecting people, not only from the numbers of people who got flu but also from looking at their T-cells before we gave them flu. The people we vaccinated had T-cells that were more activated. The people we hadn't vaccinated had T-cells as well but they were in a resting state so they would probably have taken longer to do anything. The volunteers we vaccinated had T-cells that were activated, primed and ready to kill. There were more T-cells in people we vaccinated and they were more activated." Gilbert has now sent her results to a scientific journal.

Hill said the trial proved two important things about the vaccine. "It showed that it was safe; and giving people flu virus in the presence of lots of T-cells induced by the vaccine was absolutely fine.

"The [traditional flu] vaccine efficacy is 70-80% of young people, but only 30-40% in old people," said Hill. "What we'll do is an efficacy trial in the elderly and try to improve that 30-40% to hopefully double that."

While traditional vaccines prompt the body to create antibodies, Gilbert's vaccine boosts the number of the body's T-cells, another key part of the immune system. These can identify and destroy body cells that have been infected by a virus.

In her trial, Gilbert vaccinated 11 healthy volunteers and then infected them, along with 11 non-vaccinated volunteers, with the Wisconsin strain of the H3N2 influenza A virus, which was first isolated in 2005. She monitored the volunteers' symptoms twice a day, including runny noses, coughs and sore throats, and she calculated how much mucus everyone produced by weighing tissues they used. Though a small study, it was significant in that it was the first vaccine of its type to be tested on people.

"What we'll probably do is take the existing flu vaccine and mix in the new virus-vector vaccine, so you get both good antibodies and good T-cells. As well as giving you the antibodies for this season's strain of flu, we'll give you some T-cells that will cover this season, next year, next year and thereafter. It may not be 100% effective against all strains, but at least if there were a pandemic coming around, it would cover you for any strain."

It is believed that the vaccine could provide better protection against flu for older people. The Jenner Institute scientists are already testing it on people over 50, a group that does not respond so well to traditional vaccines.

"The [traditional flu] vaccine efficacy is 70-80% of young people, but only 30-40% in old people," said Hill. "What we'll do is an efficacy trial in the elderly and try to improve that 30-40% to hopefully double that."

Gilbert says that the older people get, the less efficient their immune systems are at making new antibodies. "Immune memory lasts longer than that, so if people have already got responses to something, it's not so difficult to re-activate them. What we're trying to do with our T-cell vaccine is re-activate the T-cell responses they've already got as a result of their previous exposure to flu."

The next step for the T-cell vaccine is to stage a field trial in comparing several thousand people are given and not given the vaccine. It will take several more years, therefore, before Gilbert's vaccine can be licensed for use alongside traditional, antibody-inducing vaccines.

Mark Fielder, a medical microbiologist at Kingston University, said: "This study represents some potentially very exciting findings with positive implications not only for influenza but possibly for infectious disease in a wider context. The findings are extremely encouraging in terms of the apparent efficacy of the virus and the that it appears to be a safe formulation. However, I think that a larger trial will be able to confirm these findings and let this technology be taken forward."

He added: "T-cell vaccines are an exciting technology and we should encourage research and development into the area of vaccinology to help combat infectious diseases in all parts of the world."

A vaccine that lets a pathogen infect, but limits its severity, might be something the pathogen is not under such pressure to evade. A vaccine that mainly works through T-cells might allow the flu to pass through us every year, possibly causing little more than sniffles, but each time acting as a live booster vaccine, keeping our antibodies and other immunity to flu up to date.

Vaccines that prevent any infection at all put enormous pressure on the pathogen to evolve and evade the vaccine, as the germ cannot live otherwise - that seems to have happened with whooping cough bacteria.

several research groups are trying to make a flu vaccine out of proteins that are the same in all flu viruses, but to which people don't normally mount much of an immune response, in the hope that this will protect us from all flu once and for all.

Many are based on specific proteins of flu, called M and NP. As we reported in 2009, one of these groups, from Oxford University, are doing this using a live, innocuous strain of Vaccinia virus, which used to be used to vaccinate against smallpox.

This Ankara strain, or MVA, is especially safe and has been tested in children and the HIV-positive. Gilbert and colleagues tacked M and NP onto an MVA virus and injected it into 28 healthy adult volunteers. They report that it was safe - and had an astonishing effect on the volunteers' immune systems.

Most vaccines aim to induce antibodies, which rapidly recognise pathogens and prevent infection. MVA instead mobilises large numbers of white blood cells called T-cells. T-cells remember various bits of the germs they've seen. Virtually everyone carries a few T-cells that recognise the M and NP proteins. These cells normally help fight off flu, but aren't quick or numerous enough to prevent infection or reliably limit disease.

The Oxford vaccine stimulated huge numbers of T-cells - in fact, more than the researchers have ever seen with other experimental vaccines based on MVA and aimed at malaria, HIV and TB.